The present invention relates to a high-frequency cable of the coaxial variety, including an inner conductor, an outer conductor in concentric relation to the inner conductor, and an insulation in between.
Coaxial high-frequency cables are well known in the art. They are used, for example, for feeding a signal from a transmitter circuit to an antenna. More recently, these cables have been used increasingly in cable TV networks. Economic mass production is, therefore, of great interest to the industry.
The insulation between the inner conductor and the outer conductor does not only insulate and separate the conductors from each other, but at least a part of the insulation must serve as support for holding the inner conductor concentrically in the interior of the outer conductor, or the outer conductor must be held by the insulation, or a portion thereof, on the inner conductor.
The spacing and supporting function in such a cable is, for example, provided by means of spacer disks or by a helical element. These structures can, indeed, be made at a sufficiently high degree of accuracy so that, once the spacer or spacers have been placed on the inner conductor, the outer conductor, in turn, can be placed thereon and will assume the correct concentric position without requiring prior reworking of the spacer elements.
Another approach is to extrude a solid or foaming insulation upon the inner conductor. However, accuracy in the resulting dimensions, as far as concentricity is concerned, is not always adequate nor sufficient to provide a cable which does not exhibit reflection. Thus, such an insulation requires reworking of its surface contour to establish the desired and required concentricity of the outer surface which will support the outer conductor. It can readily be seen that such a reworking and surface finishing of the insulation surface requires cutting, i.e., removal of material. This means one has to make the insulation thicker than ultimately necessary so that one has material available for peeling, cutting, or other types of removal, to arrive at the desired diameter and circularity.
Obviously, this procedure results in the production of considerable losses and insulation scrap whose reusability is quite limited if at all possible. Moreover, gauging the insulation by such a cutting or peeling step slows production speed, and pulling the conductor through the production line and through this particular working station requires considerable force.
The British Pat. No. 767,336 discloses a method in which the insulation is, so to speak, thermally peeled. The insulated inner conductor is drawn through a heated conical die which melts the outer surface of the insulation being stripped off as excess to obtain the desired dimensions. Again, one produces a considerable amount of waste in this manner. A conical die is also used in the method as disclosed in U.S. Pat. No. 4,183,888, filed Aug. 20, 1973, but for an entirely different purpose.